Method and apparatus for synchronizing in picture-transmission systems



June 29 1926.

E. FqwAT'sou "METHOD AND APPARATUS FOR SYNCHRONIZING IN PICTURETRANSMISSION SYSTEMS U R QN.

Q l I WVVV m ma 29, 1926.- i

UNITED (STATES PATENT OFFICE.

EDWARD r. wa'rson, or naaclnlon'r, NEW 1011!, AssIcmon-ro mama rnmt-PHONE AND TELEGRAPH contra, a conroaa'non or NEW YORK.

METHOD AND APPARATUS FOR SYNCHBON I ZING IN PICTURE-TRANSMISSIONSYSTEMS.

Application filed November 28, 1925. Serial No. 71,342.

This invention relates to synchronizing and particularly to methods andmeans of securlng synchronism in systems for the transmission ofpictures and images by elec- 5 'tricity.

An object of the invention is to maintain synchronism between two widelyseparated movable elements such as the sending and receiving drums of apicture transmission system.

A feature of the invention relates to a synchronizing method wherein apicture carrier current is generated at the sending station having afrequency exactly proportional to the sending drum, and at the receivingstation a portion of such carrier is combined with a u rrent generatedat said receiving station having a frequency exactly proportional to thereceiving drum speed but slightly different from said carrier frequencyand utilizing the beat frequency current to control the speed of thereceiving ,drum.

Another feature consists in the provision of means whereby a singlecarrier wave is used both for intelligence transmission and synchronism,a portion of the carrier current of uniform amplitude being selected andcaused to beat with a locally generated current at the receiving stationfor maintaining synchronism.

Other features and advantages of the inventionwill appear from thefollowing description and also from the appended claims.

ture transmission system including a sending and a receiving station andan interconnecting transmission line. Y

i Fig. 2 illustrates the potential-current characteristic of one of thethermionic devices.

Fig. 3 illustrates the wave shape of the output current of one of thethermionic devices under one condition.

The equipment at the transmitting station, shown at the left in Fi 1,comprises a transmitting member or cy indrical drum 1 which is suitablymounted for rotation and also for movement laterally with respect to itsaxis. The drum is driven by a shaft 2 through a friction clutch 43. Bysuitable mechanical arrangements, which have not been shown but whichare well known in the In the drawing Figure 1 illustrates a pic-' 'ismaintained at this frequency by a driving magnet 5 which has its circuitinterrupted at the contact 44 carried by one of thefork tines. The othertine of the fork 4 carries contacts for completing circuits through themagnets of the phonic wheel motor 3.

The member 1, hereinafter called a drum,

is preferably a skeleton framework for carrying the film to enable thepassage of light. \Vithin the drum 1 is located an opaque stationaryshield having an aperture 46 therein. Within the shield 45 is aphotoelectric cell 10 so arranged that light passing throu h theaperture 46 impinges upon the catho e of said cell. A source of light 8is positioned at some convenient point and plfoduces a beam the size ofwhich ma be regulated by an aperture in the shiel 47. This beam isdirected and focused by the lens 9at a point which lies substantiallywithin the surface of the sendin film. The beam of light passes throught e film and the aperture 46, and impinges upon the oathode of the cell10. Thus, by wrapping a transparency of a picture around the drum 1,such transparenc may be scanned'over its entire area in a helical tracedetermined by the rotational and longitudinal movement of the drum -1.During the scanning period, ,the beam of light reaching the cathode ofthe photoelectric cell will vary in intensity in exact accordance withthe tone values of the succeeding elemental areas of the icturetransparency. The variable li ht s ining on the cell 10 causes, in thewel known manner, a variable electric current to flow in the circuit 48.This current may bev amplified by the amplifier 11 before it is used tomodulate a carrier current for transmission over the line L.

The carrier current is generated b means of an alternating currentgenerator which is gear'ed'by means of the shaft 6 directly to the shaft2 of thephonic' wheel motor 3. 7

Thus, the current generated by the generator at the-string of the lightvalve 24.

7 will have a frequency which varies directly as the speed of the motor3. The current from the machine 7 is applied to the input circuit of amodulator 12 together with the amplified picture current from theamplifier 11. Thus, the picture current serves to modulate the amplitudeof the carrier wave phonic wheel motor 19 is driven by a tuning fork 20which has a natural period the same as that of the tuning fork 4; Thefrequency of the fork 20 is maintained and regulated by means of thedriving magnet 21.

For translating the received picture currents into light variations,there is provided at the receiving station a light valve 24 of thestring galvanometer type, well known in the art. A source of light 26produces a beam the size of which maybe regulated by the aperture in ascreen'50. This beam is then directed by the lens 25 and foculs pld eaperture in the light valve 24 is in turn fo cused, by means of the lens27, at a point lying substantially within the surface of the receivingdrum l7. 1 Therefore, if a light sensitive film is wrapped about thedrum 17, and the stringv of the light valve is vibrated to vary theintensity of the beam of light passing from the source 26 to the surfaceof the sensitive film, the film may be exposed in accordance with thetone values of the pictureat the transmitting end;

To operate the light valve, the incoming modulated carrier wave isapplied to the proper amplifying devices 13 and the output current ofthese amplifiers is applied to the primary winding 14 of thetransformer. The current induced in the secondary winding 15 of thetransformer is applied to the "string of the light valve 24. Due tothemagnetic field surrounding the light valve string, this string willvibrate at an amplitude which is directly proportional to the strengthof the current flowing there through. During the vibration of the stringof valve 24:, the drum 17 is caused to rotate and move axially so thatthe beam of light of varying intensity transverses the drum 17 be drivenin exact synchronism with the drum 1 at the transmitting station.

' It is to the accomplishment of this end that the present invention isespecially directed.

plate current takes place.

At the receiving station there is provided an alternating currentgenerator 22 which, through the shaft 23, is directly geared to theshaft1'8. Thus, the speed of the generator 22 varies directly as thespeed ofthe phonic wheel motor '19 andthe shaft 18.

Accordingly, the frequency of-the current I produced by the enerator 22is a rectilinear function of t e speed of the shaft 18 The generator 22is so-designed that when the drums 1 and 17 are rotating in exactsynchronism, the frequency of the current produced by said generator isa given definite amount less than the frequency of the current producedby the generator 7. For

example, the frequencies may diii'er from 30, connected to the terminalsof the tertiary winding 16 of the transformer. A battery 31 maintainsthe grid 30 at a desired nega- L tive potential. The output circuit ofthe selector S, including the filament 28 and.

anode or plate 29, includes the space current source 32 and is connectedto the primary winding of a transformer 33.

The component circuit elements of the selector S are so selected andproportioned that the tube S is overloaded for all values of theincoming carrier current greater than This may best be seen the minimumvalue. by consulting Fig. 2. In this figure, the

curve 51 represents the incoming modulated carrier wave. The amplitudeof this carrier wave will vary between a minimum and a maximum valuedepending upon the range,

between the darkest element of the transmitted picture and the lightestelement thereof. Assume that for the ran e of intensity of pictureelements, the amp 'tude of the carrier wave never becomes less than thevalue a.

Referring now to the curve 52, this represents the grid potential-platecurrent characteristic of the tube S. It will be noted that for allvariations of grid potential between a given positive value and a givennegative value, the plate current varies along the steep portion of thecurve 52.-

F or all values, however, of grid potential greater than a given amount,the tube lee-- comes saturated and no further change in mentioned, theelements of the circuit may be so taken that this overloading is presentfor all amplitudes of the incoming wave As previously greater than theminimum amplitude. Ac-

cordingly, for an average amplitude the,

wave shape of the current in the suit of the select r S will e semen acuit of the beat detector.

lar to that shown in Fig. 3. This means that regardless of how theamplitude of the carrier wave may vary, due to the modulation by thepicture currents, the portion selected by the tube S will be convertedinto a current having an approximately uniform amplitude.

The alternating current generator 22 has its output circuit connected tothe primary of a transformer 34, the secondary of which is included inthe .input circuit of the beat detector BDQ Also, the secondary of thetransformer 33is included in the input cir- This input circuit includesthe biasing battery 38 and the filament 35 and grid 37 of the tube. Theoutput circuit of the tube BD, including the plate 36 and filament 35,is connected to the windings of a relay 41. A space current source 39-is included in series with the output circuit, and a condenser 40 isplaced in shunt of the relay 41.

Since the current flowing in the secondary of the transformer 33 has afrequency which differs from that of the current flowing in thesecondary of transformer 34, a resultant beat frequency potential willbe set up causing the current in the output circuit of the detector tohave a similiar frequency. Thus, the relay 41 will operate at a ratedetermined by the beat frequency of the two currents impressed upon theinput circuit of the beat detector. Should the receiving drum 17 var inspeed from that of the sending drum 1, t e frequency of the currentproduced by the generator 22 would vary by the same law and accordinglythe frequency of the beat frequency current would vary. For example, ifthe speed of drum 17 falls oif, the difference between the beatingfrequencies increases and accordingly the rate of operation of relay 41increases. An increase in the speed of operation of relay 41 increasesthe speed of fork 20, whereby the drum is accelerated and brought backinto synchronism with drum 1. Similarly, if drum 17 tends to run fasterthan drum 1, the beat frequency decreases in value and the frequency ofrelay 41 decreases. This acts to slow down the drum 17 to bring it backinto synchronism with drum 1. In this manner, the relay 41 may be madeto control the tuning fork 20 so as to exactly compensate or correct anytendency of speed variation.

lVhat is claimed is:

1. The method of maintaining synchronism between sending and receivingmembers which consists in' generating an alternating current having afrequency directly related to the speed of the sending-member,

generating a second alternating current hav- 1ng a frequency directlyrelated to the sfpeed of said receivin member but different rom thefrequency 0 said first current, transmit ting said first current fromthe sending memher to the receiving member, combining said currents toproduce a beat frequency cur-' rent, and utilizing said beatfrequencycurrent to govern the speed of the receiving member.

2. The method of maintaining synchro-.

ting the picture characteristics which has a frequencydetermineddirectly by the speed of the sending drum, generating a secondalternating current at the receiving station which has afrequency'determined directly by the speed of the receiving drum anddiffering from the frequency of said carrier current, transmitting thecarrier current to the receiving station, selecting from said carriercurrent a portion having a uniform amplitude regardless of the waveshape of the carrier current, combining the selected current with saidsecond mentioned current to produce a beat frequency current, andemploying said beat frequency current to govern the speed of thereceiving drum.

3. In a picture transmission system, sending and receiving memberslocated respectively at sending and receiving stations, separate shaftsfor drivingsaid members generators driven directly from said shafts forproducing alternating currents which differ from each other infrequency, means for transmitting the current generated at the sendingstation. to the receiving station, means for combining the transmittedcurrent with the current generated at the receiving station to produce abeat frequency current, and means responsive to said beat frequencycurrent for governing the speed of said receiving member.

4. In a picture transmission system, sending and receiving memberslocated respectively at sending and receiving stations, generators atsaid. sending and receiving stations, mechanical connections formaintaining adefinite relation between the speeds of said generatorsandsaid sending and receivingmembers, said generators producing alternatingcurrents respectively having frequencies which differ from each other,means for transmitting the current generated atthe sending station tothe receiving station,

quency current for governing the speed of said receiving member.

5. In a picture transmission system, sending and receiving memberslocated respectively at corresponding stations, separate shafts fordriving said members, a generator at the sending station geared directlyto the haft thereat for producing an alternating so I driven directlyfrom said shafts for produc-.

ing alternating currents which difier from each other in frequency,means for transmitting I the current generated at the sending station tothe receiving station, means at the receiving station for selecting aportion of the transmitted current, theselected current havinga uniformamplitude regardless of the variations in the wave shape or ampli tudeof the transmitted current, means for combining the selected currentwith the our rent generated at the receiving station to produce a beatfrequency current, and means responsive .to'said beat frequency currentfor governing the speed of said receiving member.

7. In a picture transmission system, sending and receiving memberslocated. respectively at corresponding stations, shafts for driving saidmembers, generators driven directly from said shafts for producingalternating currents which differ from each other in frequency, meansfor transmitting the current generated at the sending station to thereceiving station, a thermionic discharge device responsive to thetransmitted current and arranged to he overloaded at all values of saidtransmitted current for producing a current 'ofouniform amplitude, meansfor combining the current produced by said device with the currentgenerated by said generator at thereceiving station to produce aresuitant current, and means controlled by said resultant current forgoverning the speed of the receiving member.

. 8. The-combination in a picture transmission system, of sending andreceiving members located at respective stations, driving means-for saidmembers, generators niechanically connected to said driving means forgenerating alternating currents difiering mascara from each other infrequency, means for modulating the current generated at thetransmitting station in accordance with the characteristics of apicture, means for trans- I mitt-ing the modulated current to thereceiving station, means for combining a portion of the transmittedcurrent with the current generated at the receiving stat-ion to producea resultant current, and means controlled by the resultant current forcontrolling the speed of the receiving memben 9. The combination inapicture transmismeans for said members, generators mechamcally connectedrespectively to said driving means for producing alternating currentsdiffering from each other in frequency,'means for modulating the currentproduced at the sending station in accordance with the characteristicsof a picture, a transmissiop line for transmitting the modulated currentto the receiving station, a light sensitive element mounted on thereceiving member; means controlled by the modulated sion system, ofsending and receiving mem-,. bers located at respective stations,driving current for regulating the amount of light passing to said lightsensitive element, means for combining a portion of, the transmittedcurrent with the current generated at the receiving station to produce aresultant current,'and means responsive to said resultant current foroverning the speed of said receiving mem er.

10. In a picture transmission system, sending and receiving memberslocated respectively at sending and receiving stations, separate shaftsfor driving said members, a generator driven from the shaft at thesending station for producing an alternating current of a givenfrequency, means for transmitting said current to the receiving station,a generator driven from the shaft at the receiving station forgenerating an alternating current having a frequency lower than that ofsaid first-mentioned current, means for combining the transmittedcurrent with the current generated at the receiving station to produce abeat frequency current, and means responsive 'to said beat frequencycurrent for governing the speed of said receiving memlin testimonywhereof, 11 have signed my name to this specification this Qdth day ofNovember 1925. a. I

EDWARD r. WATSQN.

